1. Field of the Invention
The present invention relates in general to reproducing an electronic data signal from a received optical signal and more specifically to a method and device for extracting from the received optical signal, a timing signal for providing timing to the electric data signal reproduced from the received optical signal.
2. Description of the Related Art
Optical transmission systems with data rates of 10 Gb/s have been implemented in trunk-line optical communications. However, with rapid increases in the amount of information to be transmitted in recent years, in part resulting from the Internet, a further increase in transmission capacity is demanded. One way to achieve this is to increase transmission speeds using time-division multiplexing (including optical time-division multiplexing OTDM). Research and development for the next generation of transmission system, a 40-Gb/s system, is being carried out vigorously throughout the world. The present invention is especially for use with this next generation system.
Generally, the following two timing extraction methods have been practiced in prior known optical transmission systems to generate a clock signal from an optical signal, which clock signal has a frequency, X-GHz, nominally the same as the data transmission rate, X-Gb/s, of the optical signal.
(i) A phase locked loop (PLL) method is used when the X-GHz clock signal component is contained in the baseband spectrum of the received optical signal, such as in the case of a return-to-zero (RZ) coded signal. With the PLL method, the optical signal is first converted into an electrical signal, and then the X-GHz timing signal is extracted directly by using a band-pass filter. A voltage-controlled oscillator (VCO) outputs a clock signal. The X-GHz timing signal is phase-compared with the output of the VCO (the clock signal) for correction. Thereby, the clock signal is synchronized to the received optical signal and is generated as the output of the VCO.
(ii) A non-linear extraction method is used when the X-GHz clock component is not contained in the baseband spectrum of the received optical signal. For example, the nonlinear extraction method may be used in the case of a non-return-to-zero (NRZ) coded signal. According to the non-linear extraction method, the optical signal is first converted into an electrical signal which is then divided between two paths. The signal transmitted through one of the two paths is delayed a time equal to one-half of a one symbol period (1/40-GHz=25 ps) and then multiplied by two by introducing the signals from the two paths into an EXOR circuit. After this, the X-GHz timing signal is extracted using a band-pass filter.
In Japanese Patent Application No. 9-224056, two of the present inventors pointed out that precise dispersion compensation is essential for an ultra high-speed transmission system of 40-Gb/s or higher. As a means to achieve that end, the present inventor proposed that a timing signal component whose frequency is equal in value to the bit rate of the optical signal be extracted from the received optical signal, and that the amount of total dispersion of the optical transmission line be set so that the intensity of the timing signal frequency component becomes a maximum or a minimum.
In the case of an RZ signal, since the baseband spectrum contains a component having a frequency equal in value to the bit rate, the amount of total dispersion amount can be optimized as above using PLL (method (i)). That is, the above total dispersion optimization technique maximizes the intensity of the 40-GHz (data rate) component, and therefore the PLL method (method (i)) can be applied directly.
A 40-Gb/s OTDM signal is formed by multiplexing two optical signals modulated with two 20-GHz RZ signals opposite in phase with their tails overlapping each other with the phases of their light waves shifted 180.degree. relative to each other so as to cancel out the overlapping portions. In this case too, the 40-GHz component is contained in the baseband spectrum. Accordingly, the total dispersion amount can be optimized using method (i). That is, in the case of the OTDM signal, when the above total dispersion optimization technique is applied, the amount of total dispersion is set to minimize intensity of the 40-GHz component. However, because the intensity of the 40-GHz component does not become zero at the minimum point, method (i) can be used to generate a timing signal from the optical signal whose chromatic dispersion has been optimized by the above technique.
In the case of a non-return-to-zero (NRZ) coded signal, on the other hand, a frequency component equal in value to the signal bit rate does not exist in the baseband spectrum because of its principle of operation. Accordingly, method (i) cannot be applied, and usually the nonlinear extraction method (method of (ii)) is used. More specifically, if the above total dispersion amount optimization technique is applied to a 40-Gb/s NRZ system, the amount of total dispersion minimizes the 40-GHz frequency component, and because of its principle of operation, the intensity of the 40-GHz component becomes zero at the minimum point. Because the 40-GHz component cannot be extracted using method (i), it has been proposed to use method (ii). However, to apply method (ii) to a 40-Gb/s system requires an electronic circuit operating at 80-Gb/s, i.e., a speed two times the bit rate. This 80-Gb/s signal is to be provided at the output stage of an EXOR circuit. Circuits operating at 80-Gb/s are difficult to implement using present integrated circuit technology.